There is no general answer to your question, since music players can implement whatever they want.

However, the default players for Ubuntu all implement the mpris2 interface (if they are integrated into the soundmenu). Therefore they can be controlled via the command line.

One possible script to use is here: pympris.

You can run it like this:

mpris2-remote.py play

mpris2-remote.py stop

mpris2-remote.py next

If you prefer something more basic, you can look at the mpris2 definition here.

Alternatively you can emulate the gnome multimedia keys as you already wrote in your question.